With the rapid development of semiconductors and ultra large-scale IC, the demand for low cost isolated dc-dc converters with high current and low voltage has increased greatly. However, the conduction loss of the Schottky diode with a forward conduction voltage of approximately 0.3V has become a bottleneck in miniaturizing these converters and in improving their thermal performance. Output synchronous rectification has proven to be the only way to enhance the converter's reliability. While the gate of the synchronous rectifier needs to be oscillated by a corresponding drive circuit, the control of the drive circuit normally requires very good sequencing to prevent the cross conduction between a primary side MOSFET and a secondary synchronous rectification MOSFET.
Generally speaking, there are two ways to drive the synchronous rectifier MOSFET: self-driven and externally-driven. The self-driven scheme is widely used for its low cost and flexibility. But for flyback converters, existing self-driven circuits are often too complex to be widely adopted in practical applications. Meanwhile, the application of some self-driven converters is greatly restricted because of difficulties in controlling the cross conduction, or the driver voltage between the gate and source of the synchronous rectifier MOSFET. For example, the self-driven technology shown in FIG. 1 uses an external auxiliary winding NS1 to drive the synchronous rectifier SR2. When S1 is turned off, the voltages of the secondary windings NS1 and NS2 reverse polarity, then SR2 will be turned on and the secondary winding NS2 of the transformer will start to provide energy to the load; when S1 is turned on, the voltages of the secondary windings NS1 and NS2 will reverse polarity, and SR2 will turn off when its gate voltage is lower than its gate driver threshold. This will generate a negative pulse voltage at the gate and source of SR2. This negative voltage is in direct ratio to Vin and will ramp up significantly when Vin is high, and, in the worst case, will result in permanent damage to S1 or SR2. Even if the converter functions properly, it will be hard to improve the efficiency of the converter because the loss in the driven circuit of the synchronous rectifier will be greatly increased as the input voltage increases.
The self-driven technology shown in FIG. 2(a) is a self-driven circuit commonly used in synchronous rectifiers in flyback converters. This circuit consists of a primary power circuit, a secondary circuit, a self-driven circuit and a PWM controller. The primary circuit includes a main power MOSFET S1, the primary winding NP of the transformer and an input capacitor Cin. The secondary circuit includes the secondary winding Ns of the transformer, a rectifier SR2 and an output capacitor Cout. The self-driven circuit consists of diode D3, capacitor C2, resistor R2, an isolated drive transformer T2, capacitor C1, resistor R1 and a delay drive circuit. The delay drive circuit is made up of a delay circuit and a drive circuit. One way of forming the delay circuit is to have a diode connected with a resistor in parallel and then connected to the ground capacitor in series.
When the PWM signal from the controller changes from low to high, the signal after being operated on by the differentiating circuit comprised of R1 and C1 in series, will make the Npp end of the isolation drive transformer T2's primary winding Npp positive; hence the secondary winding Nss of T2 will be positive at the dot end, i.e. the dot-end is positive. This will turn on D3 and charge C2 up. As a result, the synchronous rectifier MOSFET SR2 will be turned off. After the PWM signal is delayed by the delay circuit, it will turn on S1 to store energy into the transformer T3. When the PWM signal changes from high to low, the Npp end of the isolated drive transformer T2's primary winding Npp will be negative at the dot-end, i.e. the dot-end is negative. This will result in the dot-end of Nss being negative. This, in turn, will result in D3 turning off causing the synchronous rectifier SR2 to be turned on. The energy stored in transformer T3 is transferred to the load through the secondary winding NS2 and the synchronous rectifier SR2. In the design of FIG. 2(a), the output self-driven circuit must have a bulky isolation drive transformer. This will make it hard to achieve high power density. On the occasions, where high voltage isolation is required for the primary and secondary windings, such a self-driven circuit will cause hindrance to the isolation. Meanwhile, because there is high leakage inductance of the self-driven transformer under this circumstances, the voltage spike of the drive wave transmitted to the secondary winding will be very high, which is very likely to break down the gate of the synchronous rectifier.